A variety of 27 to 32nm viruses have been shown to be etiologically associated with epidemic nonbacterial gastroentiritic. The Norwalk virus is the most thoroughly studied member of this group Seroepidemiologic studies using a sensitive solid phase RIA indicate: (1) Norwalk virus has a world wide distribution, (2) infections occur at a younger age in less developed countries, (3) Norwalk infection is responsible for at least 1/3rd of outbreaks of nonbacterial gastroenteritis in the U.S., (4) the majority of outbreaks are waterborne but some are associated with contaminated food or shell fish, or cannot be associated with a specific source, and (5) in less developed countries Norwalk infection may be associated with mild gastroenteristis in young children. Attempts to propogate Norwalk and other small gastroenteritis viruses have not been successful but biophysical characterization of virus derived from feces indicates that its proteins most closely resemble those of caliv=civiruses. Recently a serologically distinct 27 nm gastroenteritis virus, the Marin County virus, was successfully transmitted to adult volunteers. One volunteer developed a diarrheal illness and shed large amoounts of virus that was detected by immune electron microscopy. The Marin County virus has a buoyant density similar to Norwal virus. Serologic assays will be developed to determie its importance in outbreaks of epidemic gastroenteritis.